Method for the separation of ammonia and carbon dioxide from aqueous solutions

ABSTRACT

The present invention relates to a method for contemporaneously recovering ammonia and carbon dioxide from an aqueous solution thereof, possibly comprising their condensates, in a synthesis process of urea, characterized in that it comprises a hydrophobic microporous membrane distillation phase of an aqueous solution comprising ammonia, carbon dioxide and their saline compounds or condensates, said distillation being carried out at a temperature ranging from 50 to 250° C. and a pressure ranging from 50 KPa to 20 MPa absolute, with the formation of a residual aqueous solution, possibly comprising urea, and a gaseous permeate stream, comprising ammonia, carbon dioxide and water. The present invention also relates to an apparatus for effecting the above method and a production process of urea which comprises the above method.

The present invention relates to a method for separating ammonia andcarbon dioxide from aqueous solutions.

In particular, the present invention relates to a method forcontemporaneously recovering ammonia (NH₃) and carbon dioxide (CO₂) froman aqueous solution comprising ammonia, carbon dioxide and salinecompounds or condensates of ammonia and carbon dioxide and possiblyurea.

The need is felt for improving methods for the separation and recoveryof NH₃ and CO₂ to be used, in particular, in synthesis processes ofurea, considering the high commercial value of NH₃, and also foroptimizing a production cycle in which NH₃ and CO₂ are used as rawmaterials.

The synthesis of urea is effected by the reaction of ammonia and carbondioxide at high pressure and temperature, the subsequent separation ofthe urea from the mixture containing the non-reacted products andrecycling of the same to the synthesis reactor.

All industrial processes for the preparation of urea are based on directsynthesis according to the following reaction:

2NH₃+CO₂

CO(NH₂)₂+H₂O  (A)

This synthesis takes place in two distinct reaction steps:

NH₃+CO₂

(NH₂)COONH₄  (A′)

(NH₂)COONH₄

CO(NH₂)₂+H₂O  (A″)

In the first step (A′) an exothermic equilibrium reaction takes placehaving a high reaction rate at room temperature, which however, at thehigh temperatures required by step (A″), requires high pressures toreach a favourable equilibrium.

In the second step (A″) an endothermic reaction takes place, which onlyreaches a significant rate at high temperatures (>150° C.), with anequilibrium state which, at 185° C., starting from a mixture of reagentsin a stoichiometric ratio, leads to a CO₂ conversion slightly higherthan about 50%. This unsatisfactory conversion can be convenientlyincreased by raising the NH₃/CO₂ ratio.

Processes for the production of urea by direct synthesis starting fromammonia and carbon dioxide have been widely illustrated and described inthe specific literature of the field. A wide review of the most commonprocesses for the production of urea can be found, for example, in“Encyclopedia of Chemical Technology” Ed. Kirk-Othmer, WileyInterscience, fourth ed. (1998), Supplement, pages 597-621.

Industrial processes for the production of urea normally carry out thesynthesis in a reactor fed with NH₃, CO₂ and aqueous solutions ofammonium carbonate and/or carbamate coming from the recycled streams ofthe non-converted reagents, at temperatures ranging from 150 to 215° C.,at pressures of at least 13 MPa, with a NH₃/CO₂ molar ratio of between2.5 and 5, calculated with respect to the sum of the feeding streams,including ammonia in the form of ammonium salt. In addition to the waterformed and excess NH₃ fed, the reactor effluent still containsconsiderable quantities of CO₂, mainly in the form of non-convertedammonium carbamate.

In order to maximize the yield of the synthesis process of urea, asexplained in greater detail hereunder, the free ammonia, water andammonium carbamate contained in the effluent leaving the synthesisreactor, are separated in a series of subsequent purification steps toobtain urea containing the minimum possible quantity of reactionby-products, in particular ammonium carbamate and water. Finally, theurea, in a suitable, degree of purity, is solidified into granular form.

The ammonium carbamate is removed from the effluent leaving thesynthesis reactor by decomposition of the carbamate under suitabletemperature and pressure conditions. The ammonium carbamate is typicallydecomposed into ammonia and carbon dioxide by feeding the reactioneffluent to a high-pressure decomposer (also called stripper),substantially operating at the same pressure as the synthesis reactorand at a slightly higher temperature. The stripper consists of atube-bundle exchanger arranged vertically, in which the effluent,leaving the reactor, containing, in addition to urea, non-reactedcarbamate and ammonia in excess, is passed in a thin layer (film) alongthe inside of the tubes, whereas steam saturated at a pressure of about23 MPa is circulated and condensed in the chamber outside the tubebundle, to supply the energy necessary for the decomposition of thecarbamate and removal of the excess ammonia. In the state of the art,the stripper is also called falling film tube-bundle heat exchanger.

The gaseous ammonia, which is separated from the urea solution in thestripper, leaves the stripper entraining with it the decompositionproducts (so-called self-stripping). Alternatively, the stripping of thedecomposition products can be effected with inert gases or with ammonia,carbon dioxide or mixtures thereof, specifically introduced into thestripper.

The liquid effluent leaving the high-pressure decomposer is an aqueoussolution of urea which, however, still contains a significant quantityof carbamate and dissolved ammonia. In order to separate these compoundsfrom the urea and recover them within the process, the aqueous solutionleaving the high-pressure decomposer is fed to a second decomposer(medium-pressure decomposer) which operates at about 2 MPa and about160° C. The heat necessary for the decomposition of the carbamate inthis step is supplied by medium-pressure steam (4-5 MPa) or byrecirculating the gaseous stream containing ammonia and ammoniumcarbamate leaving the high-pressure decomposer.

The aqueous solution of urea leaving the medium-pressure decomposer issubjected to a further purification step, feeding it to a thirddecomposer (low-pressure decomposer) which operates at about 4 bar andabout 140° C. Analogously to the case of the high and medium pressuredecomposers, also in this case, the necessary heat is supplied by meansof medium-pressure steam or by the recycling of one or more hot gaseousstreams coming from other steps of the process.

In the final section of the production plant, downstream of thedecomposition sections, the aqueous solution of purified urea obtainedfrom the last decomposer is solidified into granular form in suitablegranulators or prilling towers by cooling with air.

The gaseous streams leaving the various decomposition steps (high,medium and low pressure) and containing ammonia and carbon dioxide arecondensed in suitable equipment, forming liquid streams containingammonia, carbon dioxide (prevalently in the form of ammonium carbamate)and water, which are recycled to the synthesis reactor.

If, on the one hand, the recycling of these streams increases theconversion efficiency of the raw materials consisting of ammonia andcarbon dioxide, on the other, it implies the re-introduction of waterinto the process which, considering the stoichiometry of the overallsynthesis reaction (A), significantly lowers the yield of the ureasynthesis reaction.

It is therefore evident that in processes of the type described above,in order to maximize the yield of the urea synthesis reaction, it is offundamental importance to be able to effectively and selectivelyseparate the ammonia and carbon dioxide from the aqueous solutionscontaining them, in order to recycle streams containing the lowestpossible quantity of water to the process (in particular, to thesynthesis reactor).

The use of the decomposers described above, as also that of otherseparation systems used in the state of the art (for exampledistillation columns), allows ammonia and carbon dioxide to be recoveredseparately, at a high degree of purity. The production of purecompounds, however, leads to a high energy consumption which issignificantly reflected on the overall urea production costs.

It is also known that the separation of ammonia and carbon dioxide bythe distillation of liquid streams which circulate in a production plantof urea can be affected by the formation of solid crystals in thedistillation unit, whose removal would require washing the unit withwater or another solvent with a consequent reduction in the distillationefficiency. An objective of the present invention is to overcome thedrawbacks of the known art.

A first object of the present invention relates to a method forcontemporaneously recovering ammonia and carbon dioxide from an aqueoussolution thereof, possibly comprising their condensates, in a synthesisprocess of urea, characterized in that it comprises a distillation on ahydrophobic microporous membrane phase of an aqueous solution comprisingammonia, carbon dioxide and their saline compounds or condensates, saiddistillation being carried out at a temperature ranging from 50 to 250°C. and a pressure ranging from 50 KPa to MPa absolute, with theformation of a residual aqueous solution, possibly comprising urea, anda gaseous permeate stream, comprising ammonia, carbon dioxide and water.

An object of the present invention also relates to an apparatus foreffecting the above method, comprising:

-   -   a unit for subjecting an aqueous solution comprising ammonia,        carbon dioxide and their saline compounds or condensates, to        distillation on a hydrophobic microporous membrane, with the        formation of a residual aqueous solution and a gaseous permeate        stream, comprising ammonia, carbon dioxide and water;    -   heating means of the aqueous solution comprising ammonia, carbon        dioxide and their saline compounds or condensates.

A further object of the present invention relates to a process for theproduction of urea comprising a contemporaneous recovery phase ofammonia and carbon dioxide from an aqueous solution thereof, possiblycomprising their condensates, by means of distillation on a hydrophobicmicroporous membrane of an aqueous solution comprising ammonia, carbondioxide and their saline compounds or condensates, with the formation ofa residual aqueous solution and a gaseous permeate stream, comprisingammonia, carbon dioxide and water.

In the description of the invention, object of the present patentapplication, reference is made to the following figures:

FIG. 1: schematic representation of a device for distillation on ahydrophobic microporous membrane which can be used for effecting themethod object of the present invention;

FIG. 2—schematic representation of a urea production process accordingto the state of the art;

FIG. 3—schematic representation of a urea production process which usesthe method object of the present invention, according to a firstpreferred embodiment;

FIG. 4—schematic representation of a urea production process which usesthe method object of the present invention, according to a secondpreferred embodiment.

The Applicant has surprisingly found that with the process, object ofthe present invention, based on the use of distillation on a microporousmembrane, it is possible to improve the overall conversion yield of asynthesis process of urea, at the same time reducing its energyconsumption. The distillation on a microporous membrane, in fact, allowsgaseous streams comprising ammonia, carbon dioxide and small quantitiesof water (in the form of vapour) to be effectively recovered fromaqueous solutions which circulate in this process, without the undesiredformation of solids and with an overall reduced energy consumption.

The distillation on a microporous membrane is a technique used in thestate of the art for separating gaseous compounds from solutions inwater or in organic solvents. The use of this technique however is notknown for the treatment of solutions produced within a synthesis processof urea.

In distillation on a membrane, a liquid phase and a gaseous phase areput in contact with the two opposite sides of a hydrophobic microporousmembrane. The contact between the two phases through the pores of themembrane allows a controlled mass transfer of the chemical speciespresent therein, avoiding the dispersion of one phase within the other.

The driving force of the diffusion of a chemical species from the fluidwhich flows on one side of the membrane (feeding stream) to that flowingon the opposite side (gaseous permeate stream or carrier stream) is thetemperature, concentration and pressure gradient existing between thetwo fluids.

Thanks to the high porosity of the membrane, this type of distillationprocess operates with a contact surface between the two fluids which canbe much higher with respect to that of a traditional distillation, withobvious advantages from the point of view productivity and reduction inthe encumbrance of the equipment used.

The method object of the present invention uses the distillation on ahydrophobic microporous membrane technique for contemporaneouslyrecovering ammonia and carbon dioxide from aqueous solutions in a ureaproduction process. These solutions contain, in addition to water,ammonia and carbon dioxide in the form of dissolved gases or in the formof saline compounds or condensates, such as for example ammoniumcarbamate and/or ammonium carbonate.

The method object of the present invention is preferably applied to thecontemporaneous recovery of ammonia and carbon dioxide from recycledaqueous solutions in a urea production process. The recycled aqueoussolutions are the liquid streams generated in various steps of a ureaproduction process comprising ammonia, carbon dioxide and their salts orcondensates, which can be advantageously re-fed to the synthesis reactoror to other steps of the same process in order to maximize the yield.

In a second preferred embodiment, the method object of the presentinvention can also be used for treating solutions comprising ammonia,carbon dioxide, water and urea, such as, for example the reactioneffluent leaving the synthesis reactor of the urea production process orsolutions of urea leaving the various carbamate decomposition stepswithin the same process.

Considering the operating temperature and pressure conditions adopted ina production process of urea, the term “residual aqueous solution” usedwith reference to the method object of the present invention refers tothe prevalently liquid phase which remains at the end of the membranedistillation, after the removal by evaporation of part of the speciescontained therein. The term “gaseous permeate stream”, on the otherhand, refers to the streams or mixtures in which the liquid phase issubstantially absent, due to the removal by evaporation of part of thespecies present in the aqueous solution subjected to distillation,regardless of their effective physical state.

In this case, the distillation on a hydrophobic microporous membraneunits represent a valid alternative to the tube-bundle decomposerstypically used in the high-pressure stripping step.

The above aqueous solutions which can be treated according to thepresent invention comprise ammonia, carbon dioxide, water and possiblyurea in varying ratios, depending on the step of the urea process inwhich they are produced. The above solutions preferably comprise aquantity ranging from 20 to 70% by weight of ammonia, a quantity rangingfrom 10 to 60% by weight of carbon dioxide, a quantity ranging from 10to 70% by weight of water and, possibly, a quantity ranging from 0 to60% by weight of urea.

More preferably, the above solutions preferably comprise a quantityranging from 20 to 60% by weight of ammonia, a quantity ranging from 10to 50% by weight of carbon dioxide, a quantity ranging from 10 to 60% byweight of water and, possibly, a quantity ranging from 0 to 50% byweight of urea.

The above weight percentages refer to the overall weight of ammonia orcarbon dioxide present in the solution in free form, in the form of asalt or condensate.

The method, object of the present invention, gives the best results whenapplied to aqueous solutions which form a contact angle (static) withthe material of the membrane equal to or greater than 90°, preferablygreater than 120°. The measurement of the contact angle is effected withthe known sessile drop method and is normally carried out usingautomatic optical equipment (goniometers), commercially available, whicheffect the measurement of the angle formed by a microdrop (a few μL)deposited on the surface of the solid material (membrane). The value ofthe angle 20 seconds after the depositing of the drop is considered themeasurement of the static contact angle.

In accordance with the present invention, the distillation on ahydrophobic microporous membrane is preferably carried out at atemperature ranging from 50 to 250° C. and a pressure ranging from 50KPa to 20 MPa absolute. Under these conditions, the passage takes placeof the species CO₂ and NH₃ present in the aqueous solution and watervapour through the pores of the membrane, in the form of vapour. Thequantity of water vapour in the gaseous permeate stream is in any casereduced with respect to that present in a stream obtained withtraditional distillation systems. Due to the passage of NH₃ and CO₂through the membrane, there is a progressive enrichment of the gaseouspermeate stream which flows on the side of the membrane opposite to thatof the aqueous solution and, contemporaneously, the progressivereduction in the concentration of the species present in the latter(purified aqueous solution).

The temperature and pressure conditions of the distillation are selectedin relation to the characteristics of the aqueous solution to betreated.

The distillation is preferably carried out at a temperature ranging from80 to 220° C., more preferably from 110 to 190° C., whereas the pressurepreferably ranges from 0.15 to 18 MPa absolute, more preferably from 0.5to 16 MPa absolute.

During the membrane distillation, the pressure of the gaseous streamcomprising NH₃ and CO₂, which flows on one side of the membrane (vapourside), must be kept at a value lower than or equal to the pressure ofthe solution to be treated which flows on the opposite side (liquidside). Furthermore, the difference between the pressure of the liquidside and that of the vapour side is preferably as high as possible, butmust be lower than the minimum pressure difference which would lead tothe passage of the solvent in liquid phase through the pores of themembrane (flooding), and to the subsequent mixing of the same with thegaseous stream comprising NH₃ and CO₂. This minimum pressure differenceis defined as critical pressure and can be easily determined by anexpert in the field on the basis of the characteristics of the processfluids and material forming the membrane, by possibly carrying out a fewtests and preliminary experimental measurements.

The critical pressure of the solution subjected to distillation isvariable and depends not only on the wettability characteristics of theprocessed solution (contact angle with the material forming themembrane), but also on the construction characteristics of the membraneand type of material of which the latter is composed. Critical pressuressuitable for effecting the membrane distillation step according to thepresent invention are preferably greater than 50 KPa.

The microporous membrane distillation process is preferably carried outwith devices known in the state of the art as membrane contactors.Membrane contactors (hereinafter indicated with the abbreviation “MC”)comprise any form of hydrophobic microporous membranes, such as, forexample, hollow fibres, flat membranes, spiral wound membranes, etc.

A possible preferred embodiment of an MC device is illustrated in FIG.1.

With reference to FIG. 1, the MC device 11 which can be used for thepurposes of the present invention consists of an apparatus comprising anouter casing consisting of a cylindrical mantle 12, preferably arrangedin a horizontal position during the distillation process, inside which aseries of tubular elements 13 are aligned, consisting ofcylindrically-shaped hydrophobic microporous membranes (hollow fibres),connected at the ends with a distribution chamber and a collectionchamber (the chambers are not shown in FIG. 1). The aqueous solution tobe treated for recovering ammonia and carbon dioxide contained thereinis preferably passed outside the tubular elements 13, i.e. in the spacebetween said tubular elements 13 and the cylindrical mantle 12,according to the flow direction indicated by the arrow 15. In thispreferred case, the ammonia and carbon dioxide vapours and water vapourare released, through the membranes, into the space inside the same, andare then collected through a single outlet according to the flowdirection indicated by the arrow 14.

The pressure differential between the aqueous solution flowing outsidethe tubular elements 13 and the vapours flowing in the oppositedirection inside the tubular elements 13 is conveniently maintained at avalue ranging from 40 to 150 KPa and is in any case lower than thecritical pressure which represents the wettability limit of the pores,in order to avoid the pore wetting phenomenon. As already mentioned, thecritical pressure value depends on the material of which the membrane iscomposed and the kind of solution. In order to better sustain thepressure differential, the microporous membranes can possibly besupported with a rigid material permeable to vapours (not shown in FIG.1), which, in the above preferred case, consists of a hollow tube onwhich the membranes are wound, whereas it can be a tube containing themembranes in its interior, if the aqueous solution is passed inside thetubular elements 13 of the MC device 11.

In the embodiment illustrated in FIG. 1, the flow of aqueous solution isin countercurrent with respect to that of the gaseous permeate stream.In the method according to the present invention, however, the twostreams can also flow cocurrently. Furthermore, the method can also beapplied in batch mode.

Heat can be supplied to the device 11, for example by means of medium-or high-pressure steam, which passes through a tube bundle or heatingchamber (not shown in the figure) subsequently exiting from the device11 in a condensate phase. More conveniently, however, the necessary heatcan be provided by suitably irradiating the aqueous solution in contactwith the microporous membranes with a beam of microwaves 16 at afrequency within the range of 2,300 to 2,700 MHz, preferably from 2,400to 2,600 MHz. The most suitable frequencies for an optimum absorption ofthe polar molecules present in the solution treated can be easilyselected by an expert in the field in relation to the composition andtemperature of the solution, on the basis of the absorptioncharacteristics specified in literature or simple preliminary scanningtests. In this case, the rigid materials of which the apparatus iscomposed (cylindrical mantle 12 and possible cylindrical supports madeof a material permeable to vapours), must be selected from thosetransparent to microwaves within the frequency range used.

The microporous membrane distillation is preferably effected underself-stripping conditions, i.e. in the absence of an additional carrierstream. In order to obtain a greater extraction of CO₂ and NH₃, a streamof CO₂ and/or NH₃, and possibly a stream of inert gas can be preferablyused as carrier stream, maintained at a pressure close to, but lowerthan, that of the aqueous solution treated (feeding stream).

The membranes used for the purposes of the present invention typicallyconsist of materials based on hydrophobic polymers, which form a contactangle≧90°, preferably ≧120° with the processed solutions, as previouslyspecified. In a first approximation, for the purposes of the presentinvention, the hydrophobicity of the membranes can be evaluated on thebasis of these criteria applied to the measurement of the contact anglewith water, rather than with the process solution.

Examples of suitable materials for forming the membranes are fluorinatedpolymers and copolymers, such as polytetrafluoroethylene (PTFE),polyvinylidenefluoride or Nafion®, certain polyolefins with a highcrystallinity, such as isotactic polypropylene, polyacrylonitrile,polysulfones. These materials offer a high thermal resistance (up to220-250° C.), and a high chemical and mechanical resistance.

The maximum pressure difference which can be sustained by thesemembranes is about 100 KPa. This kind of membrane is commerciallyavailable.

In a preferred embodiment, the distillation on membrane is carried outin distillation units containing two or more MC devices of the typedescribed above which can operate under different temperature andpressure conditions.

In the process according to the present invention, the distillationtemperature is preferably maintained at 50 to 250° C. by irradiationwith electromagnetic radiations having a frequency within the microwaverange. Even more preferably, the irradiation of the aqueous solution iseffected so that the temperature of the aqueous solution increases alongthe flow direction of the feeding stream, i.e. lower at the inlet of theMC device containing the membrane and higher at the outlet. As thedistillation process proceeds, the aqueous solution becomes impoverishedin NH₃ and CO₂ and the liquid-vapour equilibrium conditions become lessfavourable for the separation of these species from the aqueous solutionand their passage through the membrane. Upon heating the aqueoussolution to an increasing temperature as it flows in contact with themembrane, the reduced tendency of the ammonia and carbon dioxide toseparate from the aqueous solution is compensated, also avoiding thecondensation of the vapours of the gaseous permeate stream containingammonia, carbon dioxide and water in the vapour side.

The heating can be effected with microwave generation devices known inthe state of the art. The use of microwaves offers the advantage ofbeing able to selectively heat only the molecules of water, NH₃, CO₂ andother polar molecules in the liquid state, avoiding a significantheating of those present in vapour form. This enables thermal energy tobe supplied to the feeding stream, favouring the subsequent permeationof gaseous NH₃ and CO₂ through the membrane.

Furthermore, the use of microwaves also prevents the flooding of thepores of the membrane, i.e. the penetration into the pores of thesolvent (water) of the aqueous solution in liquid form. Should floodingof the pores occur due to an overpressure on the side of the membrane inwhich the solution to be treated flows, the selective heating of themolecules of liquid water on the part of the microwaves allows theliquid water which has penetrated inside the pores, to evaporate, thusregenerating the membrane in situ without interrupting the functioningof the equipment and avoiding the application of counterpressures.

Furthermore, it has been found that the use of microwaves for heatingthe liquid has the further advantage of favouring the dispersion of theammonia and carbon dioxide in the possible gaseous carrier stream,without causing its undesired heating and increasing the energyconsumption.

In order to supply heat by irradiation with microwaves, MC devices mustbe used in which the outer casing consists of materials transparent tomicrowaves, for example materials such as PTFE, glass, Pyrex, etc.

Heating by means of microwaves is not only easy to apply but also allowsthe thermal energy supplied to the membrane distillation device to beaccurately modulated. Microwave heating devices, moreover, havingconversion efficiencies of electric energy into microwaves in the orderof about 70%, contribute to obtaining an overall higher energy yield ofthe recovery process of ammonia and carbon dioxide.

Although heating with the use of microwaves is preferred for theadvantages described above, the heating of the aqueous solution can alsobe effected with the conventional techniques, for example by passing theaqueous solution in a heat exchanger, before subjecting it todistillation on membrane.

The main operating parameters which influence the distillation onmembrane according to the method, object of the present invention, arethe flow rate and pressure of the aqueous solution in contact with themembrane, the flow rate and pressure of the gaseous permeate stream andpossible additional carrier stream, as well as the temperature gradientobtained in the aqueous solution by irradiation with microwaves. Thedegree of influence on the effectiveness of the method of each of theabove operating parameters can be evaluated experimentally. Optimumoperating conditions are obtained when the flow rate of the aqueoussolution and its pressure on the surface of the membrane is such as tocompensate the pressure drop due to the passage through the membrane ofthe gaseous species leaving the aqueous solution.

The distillation on membrane of an aqueous solution containing ammonia,carbon dioxide and water according to the method, object of the presentinvention, returns a gaseous permeate stream and a purified aqueoussolution having variable characteristics in relation to the compositionof the solution treated and operating conditions at which thedistillation on membrane is effected. The distillation of solutionscomprising ammonia, for example, in a quantity ranging from 5 to 400,preferably from 10 to 40% by weight, carbon dioxide in a quantityranging from 2.5 to 20%, preferably from 5 to 20% by weight and urea ina quantity ranging from 10 to 60%, preferably from 20 to 40% by weight,effected at a temperature ranging from 100 to 220° C. and a pressureranging from 10 to 18 MPa, produces a purified aqueous solution having aresidual ammonia content ranging from 1 to 30% by weight, carbon dioxideranging from 1 to 10% by weight, urea in a quantity ranging from 20 to60%, preferably from 30 to 60%, by weight.

According to a different aspect of the process of the present invention,relating to the concentration of recycled solutions without urea andcomprising ammonia in a quantity ranging from 5 to 70%, preferably from20 to 70% by weight, carbon dioxide in a quantity ranging from 2.5 to30%, preferably from 10 to 30% by weight, the distillation is carriedout at a temperature ranging from 60 to 200° C., preferably from 80 to180° C. and a pressure ranging from 1 to 10 MPa, a purified aqueoussolution is obtained having ammonia and carbon dioxide contents greatlyreduced with respect to the initial solution, respectively ranging up to20% by weight of NH₃ and up to 10% by weight of CO₂, but preferablylower than 2%, more preferably lower than 1%, by weight.

As the gaseous permeate stream obtained with the method, object of thepresent invention, substantially consists of NH₃ and CO₂ and with a lowwater content, it can be recycled to the urea synthesis reactor,possibly after recovery of the residual heat, or to another productionprocess (for example, in a synthesis process of ammonia).

The gaseous permeate stream can also be subjected to further separationprocesses to recover NH₃ and CO₂ separately, with a high degree ofpurity.

The method according to the present invention allows the contemporaneousrecovery of NH₃ and CO₂ contained in an aqueous solution in a ureaproduction process, allowing the recovery of the above compounds. Themethod is therefore characterized by a high energy efficiency and doesnot have the problems of the undesired formation of solid impurities ofthe techniques of the state of the art.

The method according to the present invention also has a high energyefficiency, particularly marked in the case of the preferred use ofmicrowaves for the heating of the membrane distillation devices, and bythe fact that the ammonia and carbon dioxide can be contemporaneouslyrecovered in a single recyclable gaseous stream (as such, or as aconcentrated solution of ammonium carbonate or carbamate, aftercondensation) to the urea synthesis reactor.

Finally, the process according to the present invention has thefollowing further advantages, with respect to the techniques used in thestate of the art deriving from the use of membrane distillation:

-   -   a high separation efficiency of the ammonia and carbon dioxide        also in diluted solutions, due to the fact that, as the        interface surface consists of pores of the membrane, it does not        vary with a variation in the flow conditions of the carrier        stream and feeding stream;    -   absence of the formation of emulsions as there is no dispersion        phenomenon between the fluids;    -   absence of corrosion phenomena of the devices used for the        distillation on membrane, thanks to the particular type of        materials used for the membranes and for the casing of the MC        devices;    -   the fluids in contact with the membrane do not need to have a        different density;    -   the scale-up operations of the membrane distillation processes        are simplified, as an increase in the volume of the feeding        stream to be treated corresponds to a linear increase in the        number of modules (MC devices);    -   there is no transfer by evaporation of part of the solution to        be treated or treated in the gaseous stream containing ammonia        and carbon dioxide possibly fed to the urea plant: in this way,        the urea plant is not polluted with substances foreign to the        process itself;    -   there are no moving mechanical parts subject to wear or possible        breakage;    -   reduced encumbrance of the equipment necessary for the        distillation.

Some application examples to a urea production process are illustratedhereunder to outline the characteristics and advantages of the method ofthe present invention. The examples are provided for purely illustrativepurposes of the present invention and should not be considered aslimiting the protection scope defined by the enclosed claims.

EXAMPLE 1 Comparative

FIG. 2 illustrates a schematic representation of a process for theproduction of urea according to the state of the art. Particularfunctions such as pumps, valves, and other equipment not significant fora full understanding of the processes schematized, are not shown in theabove FIG. 2.

According to a process known in the state of the art, 2,366 tons/day ofa stream 1 of ammonia and ammonia carbamate consisting of 1,564 tons/dayof ammonia, 498 tons/day of carbon dioxide and 304 tons/day of water,are fed to a reactor R. The feeding stream 1 is obtained by mixing arecycled stream 2 (1,575 tons/day) consisting of an aqueous solution ofammonium carbamate leaving a high-pressure condenser/separator C1 and astream 3 of liquid ammonia (790.6 tons/day) substantially pure (99.91%ammonia, 0.096 water) leaving a condensation/storage section CS ofammonia. 736.3 tons/day of a stream 4 of carbon dioxide are also fed tothe reactor R. The reactor R operates at a temperature of 187° C. and apressure of 15.8 MPa (abs).

A liquid stream 5 (reaction effluent) comprising urea, carbon dioxide,water and non-reacted ammonia leaves the reactor R. The reactioneffluent (stream 5) is a stream of 3,102 tons/day consisting of 995tons/day of ammonia, 498 tons/day of carbon dioxide (prevalently in theform of ammonium carbamate), 605 tons/day of water and 1,004 tons/day ofurea. The reaction effluent (stream 5) is fed to a first high-pressuredecomposer D1 (stripper) consisting of a falling film tube bundle heatexchanger, operating at a temperature of 204° C. and a pressure of 14.7MPa (abs). The decomposer D1 is heated by feeding a stream of saturatedsteam (644 tons/day) into the mantle at a pressure of about 2.3 MPa(abs).

In the decomposer D1, the ammonium carbamate is decomposed to ammoniaand carbon dioxide, in accordance with the thermodynamic equilibriumwhich is established under the specific temperature and pressureconditions at which the first decomposer D1 is operating. A gaseousstream 6 (811 tons/day) containing ammonia (432 tons/day), carbondioxide (337 tons/day) and water vapour (42 tons/day) leaves the head ofthe first decomposer D1, which is fed to a condenser/separator C1(high-pressure condenser/separator), operating under substantiallyisobar conditions with respect to those of the decomposer D1, in orderto yield its own residual heat. An aqueous solution of urea 7 (2,291tons/day) containing ammonia (563 tons/day), carbon dioxide (161tons/day), water (563 tons/day) and urea (1,004 tons/day), leaves thebottom of the decomposer D1. The non-condensed fraction of the gaseousstream 6 fed to the high-pressure condenser/separator C1 is, on theother hand, separated (stream 8) and fed to a second decomposer D2. Thestream 8 (14 tons/day) fed to the second decomposer D2 consists ofammonia (13 tons/day), carbon dioxide (1 ton/day) and is substantiallyfree of water.

The solution of urea 7 leaving the first decomposer D1 is fed to asubsequent decomposition step of the carbamate in the second decomposerD2 (medium-pressure decomposer) which operates at about 160° C. and apressure of about 2 MPa (abs), with the separation of a gaseous stream 9(741 tons/day) containing ammonia (475 tons/day), carbon dioxide (131tons/day) and water vapour (135 tons/day), which is fed to acondenser/absorber C2 (medium-pressure condenser/absorber), forcondensation and contemporaneous recovery of the residual heat.

The solution comprising urea leaving the second decomposer D2 (stream10—1,564 tons/day) consists of ammonia (101 tons/day), carbon dioxide(31 tons/day), water vapour (428 tons/day) and urea (1,004 tons/day).The above stream 10 is fed to the block PG, i.e. the subsequent processsections in which further decomposition phases of the residual carbamateand condensation/separation of the gaseous products are effected,together with the purification and solidification phases to obtain thefinal solid product consisting of urea in granules and the treatmentphase of the wastewater. In the process phases represented by the blockPG of FIG. 2, further gaseous and liquid streams are produced,comprising ammonia, carbon dioxide and water, which are recycled to thereactor R. The recycled stream coming from the block PG is representedin FIG. 1 by a stream 11 (252 tons/day) which connects the block PG withthe section C2. The above recycled stream 11 contains ammonia (101tons/day), carbon dioxide (31 tons/day) and water vapour (120 tons/day).

In the section C2, the gaseous stream 9 leaving the second decomposer D2is partially condensed and joined with the recycled stream 11 comingfrom the block PG, with the formation of a stream 12 (778.1 tons/day)containing ammonia (355 tons/day), carbon dioxide (162 tons/day) andwater (261.1 tons/day). In the section C2, a stream 13 of gaseousammonia is also separated, which is fed to the condensation/storagesection CS of ammonia.

In the section C1, high-pressure condenser/separator, the gaseous stream6 coming from the decomposer D1 is partially condensed and joined withthe stream 12, leaving the section C2, with the formation of a stream 2(1,575 ton/day) consisting of ammonia (774 tons/day), carbon dioxide(498 tons/day) and water vapour (303 tons/day).

In the process described above, in order to produce 1,004 tons/day ofurea, a quantity of saturated steam at 2.3 MPa(abs), equal to 644 tons,was introduced into the decomposer D1. The urea synthesis reaction hadan actual yield equal to 60%.

EXAMPLE 2

The synthesis process of urea was carried out in the same plantdescribed in Example 1, applying the process object of the presentinvention for recovering ammonia and carbon dioxide from the effluentleaving the synthesis reactor. The operating conditions of the process,where not specifically indicated, are identical to those described inExample 1.

The modified process is schematically illustrated in FIG. 3, in whichthe symbols used, when coinciding with those of FIG. 1, have the samemeaning indicated in Example 1.

In the synthesis process of urea represented in FIG. 3, 2,136 tons/dayof a stream 1 of ammonia and ammonia carbamate consisting of 1,518tons/day of ammonia, 404 tons/day of carbon dioxide and 214 tons/day ofwater, are fed to a reactor R. The stream 1 is obtained by mixing arecycled stream 2 (1,345 tons/day) consisting of an aqueous solution ofammonium carbamate leaving a high-pressure condenser/separator C1 and astream 3 of liquid ammonia (790.6 tons/day) substantially pure (99.91%ammonia, 0.09% water) leaving a condensation/storage section CS ofammonia. 736.3 tons/day of a stream 4 of carbon dioxide are also fed tothe reactor R.

The reaction effluent 5 (2,872 tons/day) leaving the reactor R consistsof a stream of ammonia (949 tons/day), carbon dioxide (404 tons/day),water vapour (515 tons/day) and urea (1,004 tons/day). The effluent 5 isfed to a distillation on hydrophobic microporous membrane unit UDM,operating under the same temperature and pressure conditions as thedecomposer D1 of Example 1. The heating of the UDM unit is effected bymeans of an irradiation device with microwaves. The distillation iscarried out under self-stripping conditions.

A first gaseous stream 6 (645 tons/day) containing ammonia (353tons/day), carbon dioxide (270 tons/day) and water vapour (22 tons/day)leaves the UDM unit, and is fed to a high-pressure condenser/separatorC1. An aqueous solution of urea 7 (2,227 tons/day) containing ammonia(596 tons/day), carbon dioxide (134 tons/day), water (515 tons/day) andurea (1,004 tons/day) leaves the bottom of the UDM unit.

The non-condensed fraction of the gaseous stream 6 is separated and fed(stream 8) to a second decomposer D2. The stream 8 (12 tons/day)consists of ammonia, carbon dioxide (1 ton/day) and is substantiallyfree of water.

The solution of urea 7 leaving the first decomposer UDM is fed to asubsequent decomposition step of the carbamate in a decomposer D2(medium-pressure decomposer), with the separation of a further gaseousstream 9 (705 tons/day) containing ammonia (512 tons/day), carbondioxide (110 tons/day) and water vapour (83 tons/day), which is fed to acondenser/absorber C2 (medium-pressure condenser).

The solution comprising urea leaving the decomposer D2 stream 10 (1,534tons/day) consists of ammonia (95 tons/day), carbon dioxide (25tons/day), water vapour (410 tons/day) and urea (1,004 tons/day). Thestream 10 is fed to the subsequent process phases schematicallyrepresented by the block PG. A stream 11 (222 tons/day) is recycled fromthe block PG, which is fed to the section C2 and contains ammonia (95tons/day), carbon dioxide (25 tons/day) and water vapour (102 tons/day).

In the section C2, the gaseous stream 9 leaving the decomposer D2 ispartially condensed and joined with the recycled stream 11 coming fromthe block PG, with the formation of a stream 12 (712.1 tons/day)containing ammonia (386 tons/day), carbon dioxide (135 tons/day) andwater (191.1 tons/day). In the section C2, a stream 13 of gaseousammonia is also separated, which is fed to the condensation/storagesection CS of ammonia.

In the section C1, high-pressure condenser/separator, the gaseous stream6 coming from the first decomposer UDM is partially condensed and joinedwith the stream 12, leaving the section C2, with the formation of astream 2 (1,345 ton/day) consisting of ammonia (728 tons/day), carbondioxide (404 tons/day) and water vapour (213 tons/day).

The use of the method, object of the present invention, allowed theselective separation of ammonia and carbon dioxide to be obtained,avoiding the formation of solid products and considerably reducing theundesired transfer of water in the form of vapour. Under these processconditions, in order to produce 1,004 tons of urea, a quantity ofenergy, in the form of microwave radiations, equivalent to 590 tons ofsaturated steam at 2.3 MPa(abs), was supplied to the UDM unit, with asaving of 54 equivalent tons with respect to the process of Example 1.In the urea synthesis reaction an actual yield equal to 65% was reached.

EXAMPLE 3

The synthesis process of urea was carried out in the same plantdescribed in Example 1, applying the process object of the presentinvention for recovering ammonia and carbon dioxide from a recycledstream leaving the medium-pressure condenser. The operating conditionsof the process, where not specifically indicated, are identical to thosedescribed in Example 1.

The modified process is schematically illustrated in FIG. 4, in whichthe symbols used, when coinciding with those of FIG. 1, have the samemeaning indicated in Example 1.

In the synthesis process of urea represented in FIG. 4, 1,999 tons/dayof a stream 1 of ammonia and ammonia carbamate consisting of 1,520tons/day of ammonia, 350 tons/day of carbon dioxide and 129 tons/day ofwater, are fed to a reactor R. The stream 1 is obtained by mixing arecycled stream 2 (774 tons/day) consisting of an aqueous solution ofammonium carbamate leaving a high-pressure condenser/separator C1 and astream 3 of liquid ammonia (1,225 tons/day) substantially pure (99.91%ammonia, 0.09% water) leaving a condensation/storage section CS ofammonia. A stream 4 (736.3 tons/day) of carbon dioxide is also fed tothe reactor R.

The reaction effluent 5 leaving the reactor R is a stream of 2,735tons/day consisting of 951 tons/day of ammonia, 350 tons/day of carbondioxide, 430 tons/day of water and 1,004 tons/day of urea. The reactioneffluent 5 is fed to a first high-pressure decomposer D1 consisting of afalling film tube-bundle heat exchanger. In the decomposer D1, theammonium carbamate is decomposed to ammonia and carbon dioxide creatinga gaseous stream 6 (504 tons/day), which leaves the head of thedecomposer D1. The above gaseous stream 6, containing ammonia (281tons/day), carbon dioxide (205 tons/day) and water vapour (18 tons/day),is fed to a high-pressure condenser/separator C1, which operates undersubstantially isobar conditions with respect to those of the firstdecomposer D1, in order to yield its residual heat. An aqueous solutionof urea 7 (2,231 tons/day) containing ammonia (670 tons/day), carbondioxide (145 tons/day), water (412 tons/day) and urea (1,004 tons/day)leaves the bottom of the first decomposer D1. The non-condensed fractionof the gaseous stream 6 (stream 8) is fed to a second decomposer D2. Thestream 8 (17 tons/day) consists of ammonia (2 tons/day), carbon dioxide(15 tons/day) and is substantially free of water.

The solution of urea 7 leaving the first decomposer D1 is fed to asubsequent decomposition step of the carbamate in the second decomposerD2 (medium-pressure decomposer). In this decomposer D2, the separationtakes place of a gaseous stream 9 (750 tons/day) containing ammonia (529tons/day), carbon dioxide (130 tons/day) and water vapour (91 tons/day),which is fed to a condenser/absorber C2 (medium-pressure condenser).

The solution comprising urea leaving the decomposer D2 stream 10 (1,498tons/day) consists of ammonia (143 tons/day), carbon dioxide (30tons/day), water vapour (321 tons/day) and urea (1,004 tons/day). Theabove stream 10 is fed to the subsequent process phases schematicallyrepresented by the block PG. A stream 11 (275 tons/day) is recycled fromthe block PG, which is fed to the medium-pressure condenser C2 andcontains ammonia (146 tons/day), carbon dioxide (34 tons/day) and watervapour (95 tons/day).

In the medium-pressure condenser C2, the gaseous stream 9 leaving thesecond decomposer D2 is partially condensed and joined with the recycledstream 11 coming from the block PG, with the formation of a stream 12(836 tons/day) consisting of an aqueous solution containing ammonia (480tons/day), carbon dioxide (164 tons/day) and water (192 tons/day). Inthe section C2, a stream 13 of gaseous ammonia is also separated, whichis fed to the condensation/storage section CS of ammonia.

The stream 12 leaving the medium-pressure condenser C2 is fed to adistillation on a hydrophobic microporous membrane unit UDM, consistingof a first section S1, operating at a pressure of about 2 MPa, and asecond section S2, operating at a pressure of about 3 MPa. The heatingof the distillation unit UDM is effected, in both sections, by microwaveirradiation devices. The distillation is carried out in both sectionsunder self-stripping conditions.

In the first section S1, a gaseous permeate stream is separated,prevalently consisting of ammonia (stream 1613 460 tons/day), which issent to the condensation/storage section CS, and a stream 17, consistingof a solution containing ammonia, carbon dioxide and water. The stream17 leaving the first section S1 of the UDM unit is subjected todistillation on a microporous membrane in the second section S2, aftermixing with a stream of water 18 (200 tons/day) coming from the blockPG. Considering the behaviour of the ternary ammonia, carbon dioxide andwater mixtures, the addition of a further quantity of water to thespecific aqueous solution consisting of the stream 17 leaving the firstsection S1, allows a composition of the aqueous solution to be reachedwhich is such that in the second section S2 a liquid stream 19 (residualaqueous solution—289 tons/day) is separated, prevalently containingwater (282 tons/day) and smaller quantities of ammonia (3 tons/day) andcarbon dioxide (4 tons/day), and a stream 20 (gaseous permeatestream—287 tons/day) containing most of the carbon dioxide (160tons/day), in addition to ammonia (17 tons/day) and water (110tons/day).

The liquid stream 19 leaving the second section S2 is recycled to theurea process phases represented by the block PG, whereas the stream 20leaving the second section S2 of the UDM unit is fed to thehigh-pressure condenser/separator C1. In the above condenser/separatorC1, the gaseous stream 6 coming from the decomposer D1 is partiallycondensed and joined with the above stream 20, with the formation of thestream 2 (774 tons/day), sent to the reactor R, consisting of ammonia(296 tons/day), carbon dioxide (350 tons/day) and water vapour (128tons/day).

The use of the method, object of the present invention, allowed theselective separation of ammonia and carbon dioxide to be obtained,avoiding the formation of solid products and considerably reducing theundesired transfer of water in the form of vapour. Under these processconditions, in order to produce 1,004 tons of urea, a quantity ofsaturated water vapour 2.3 MPa (abs) equal to 550, was supplied to thefirst decomposer D1, with a saving of 94 tons with respect to theprocess of Example 1. In the urea synthesis reaction an actual yieldequal to 68% was reached.

1. A method for the contemporaneous recovery of ammonia and carbondioxide from an aqueous solution thereof, in a urea synthesis process,the method comprising distilling on a hydrophobic microporous membrane afirst aqueous solution comprising ammonia, carbon dioxide and,optionally, their saline compounds or condensates, wherein thedistilling is performed at a temperature of 50 to 250° C. and at apressure of 50 KPa to 20 MPa absolute, with the formation of to obtain aresidual aqueous solution, optionally comprising urea, and a permeatedgaseous stream comprising ammonia, carbon dioxide and water.
 2. Themethod to of claim 1, wherein the first aqueous solution is a recycledsolution coming from a urea production process or an effluent comingfrom a urea synthesis reactor.
 3. The method of claim 1, wherein thefirst aqueous solution comprises 20 to 70% by weight of ammonia, 10 to60% by weight of carbon dioxide and 10 to 70% by weight of water.
 4. Themethod of claim 1, wherein the first aqueous solution further comprises0 to 60% by weight of urea.
 5. The method of claim 1, wherein thedistilling is performed at a temperature of 80 to 220° C.
 6. The methodof claim 1, wherein the first aqueous solution comprises 5 to 40% byweight of ammonia, 2.5 to 20% by weight of carbon dioxide, and 10 to 60%by weight of urea, and the distilling is performed at a temperature of100 to 220° C. and a pressure of 10 to 18 MPa.
 7. The method of claim 1,wherein the first aqueous solution comprises no urea and comprises 5 to70% by weight of ammonia and 2.5 to 30% by weight of carbon dioxide, andthe distilling is performed at a temperature of 60 to 200° C. and apressure of 1 to 10 MPa.
 8. The method of claim 1, wherein the firstaqueous solution is heated by irradiation with microwaves.
 9. The methodof claim 8, wherein the irradiation of the first aqueous solution isperformed in such a way that a temperature of the aqueous solutionincreases along a direction of a stream of aqueous solution.
 10. Themethod of claim 1, wherein the first aqueous solution forms a contactangle with a material of the membrane equal to or greater than 90°. 11.The method of claim 1, further comprising recycling the permeatedgaseous stream within the urea synthesis process.
 12. The method ofclaim 1, wherein the distilling is performed in the presence of anadditional carrier stream.
 13. Equipment for effecting the method ofclaim 1, comprising: a distillation unit comprising a hydrophobicmicroporous membrane, wherein the distillation unit is suitable fordistilling a first aqueous solution comprising ammonia, carbon dioxideand, optionally, their saline compounds or condensates, with theformation of a residual aqueous solution and a permeated gaseous streamcomprising ammonia, carbon dioxide and water; a heating unit suitablefor heating the first aqueous solution.
 14. The equipment of claim 13,wherein the heating unit comprises a microwave-generating device. 15.The equipment of claim 13, wherein the distillation unit comprises twoor more sections which can operate under different temperature andpressure conditions.
 16. A production process for producing urea,comprising a contemporaneous recovery of ammonia and carbon dioxide froma first aqueous solution, by distillation on a hydrophobic microporousmembrane of the first aqueous solution comprising ammonia, carbondioxide and, optionally, their saline compounds or condensates, with theformation of a residual aqueous solution and a permeate gaseous streamcomprising ammonia, carbon dioxide and water.
 17. The method of claim 1,wherein the distilling is performed at a temperature of 110 to 190° C.18. The method of claim 1, wherein the distilling is performed at apressure of 0.15 to 18 MPa absolute.
 19. The method of claim 1, whereinthe distilling is performed at a temperature of 80 to 220° C. and apressure of 0.15 to 18 MPa absolute.
 20. The method of claim 1, whereinthe distilling is performed at a temperature of 110 to 190° C. and apressure of 0.5 to 16 MPa absolute.